The present invention relates to a magneto-optical recording medium and, more particularly, to a magneto-optical recording medium having a recording layer made of a rare-earth-transition metal amorphous ferrimagnetic alloy having an axis of easy magnetization perpendicular to a surface of the recording film.
As a promising data-rewritable recording medium, an optical disk having a magneto-optical recording layer is now attracting much attention. A recording layer of the data-rewritable optical disk is made of a rare-earth-transition metal amorphous ferrimagnetic alloy (hereinafter called "RE-TM film"). When a recording magnetic field is applied to the recording layer and a laser beam is focused on the recording layer, the perpendicular magnetization in the RE-TM film is inverted in accordance with the light intensity, and bit data is magnetically stored. In order to read out storage data, the recording layer of the disk is irradiated with a DC-polarized reproduction laser beam. The data is read out by utilizing rotation of polarization plane of light transmission occurring in the recording layer
In an optical disk having a recording layer comprising RE-TM film, the uniform recording layer can be relatively easily formed on a large-sized disk substrate using an industrially advantageous method of forming film forming such as sputtering or deposition. Since RE-TM film can be formed on a disk substrate without heating the substrate, a resin such as polymethylmethacrylate or polycarbonate can be employed as a substrate material.
Despite the above advantages, a recording layer comprising RE-TM film, can hardly achieve a sufficiently high carrier-to-noise ratio in data reproduction. This problem is due to a polar Kerr rotation angle of the RE-TM film itself not being sufficiently large.
In order to resolve the above problem, a transparent thin film is, conventionally, stacked on one surface of the RE-TM recording film, to which the reproduction light beam is applied. In general, the disk substrate of a recording medium of this type is transparent, and the reproduction light beam is applied to the recording film through the transparent substrate. Therefore, a transparent multi-interference layer is formed between the substrate and the RE-T recording film. The multi-interference layer increases the polar Kerr rotational angle of the RE-TM recording film. Thus, the carrier-to-noise ratio of the magneto-optical recording disk can be improved.
However, when a Kerr hysteresis characteristic of the magneto-optical recording disk having the multi-interference layer was actually determined, it was found that rise and fall of the hysteresis loop were moderate, and far from the ideal hysteresis loop shape (i.e., a rectangular shape in which rise and fall are sharp). This means that in the RE-TM recording film, magnetization inversion is slowly performed in accordance with a change in magnetic field H. When a magneto-optical recording medium having a recording film with such an irregular hysteresis loop is used as a memory, the storage bit data is changed. A stable data storage cannot be achieved.